Electrodeposition of chromium



Patented May 9, 1950 ELECTRODEPOSITION CHROMIUM Rex R. Lloyd, BoulderCity, Nev., assignor, by

mesne assignments, to Crimora Research & Development Corporation, NewYork, N. Y., a corporation of Virginia No Drawing. Application February4, 1946, Serial No. 645,473

1 Claim.

The present invention relates to the electrodeposition of chromium and,more particularly, to the electrowinning of chromium from chromium saltsolutions. This application is a continuation in part of applicationsSer. Nos. 578,549 and 578,551, filed February 17, 1945, now abandoned.

Heretofore, it has not been .possible to recover chromium economicallyfrom chromium salt baths by electrolytic methods. For example, the useof prior methods generally has resulted in thin, usually oxidecontaminated deposits at low current efficiency.

In accordance with the present invention, it is possible to plate outthick deposits of chromium of good quality which can be readily strippedfrom the cathode.

The present invention also makes it possible to deposit chromiumuniformly and satisfactorily at a high current efficiency.

An important aim of the invention is to provide a method ofelectrowinning chromium which may be carried out continuously overlongperiods of time without materially decreasing the quality of thedeposit, while maintaining a high current efficiency.

Other aims and advantages of the invention will be apparent from thefollowing description of the invention and the appended claim. i Ingeneral, the present invention is applicable to the electrowinning ofchromium from a solution containing ammonium, sodium, sulfate, chromousand chromic ions. The invention resides in the discovery that the newand improved results referred to above may be obtained by controllingthe conditions of electrolysis within the ranges or limits defined moreparticularly hereinafter. Such conditions include, for example, thecomposition, purity, temperature and pH of the bath, and the currentdensity used.

In the practice of the present invention, it is preferred, but notessential, to use an electrolytic cell divided into catholyte andanolyte compartments by means of an ion-permeable diaphragm, which typeof apparatus is well known in the art. With respect to the electrodes,it is preferred to use lead or lead-alloy anodes while the cathodespreferably are of bronze containing about 88% copper and the ibalancetin. The particular composition of the electrodes is not critical. Withrespect to the anodes, for instance, it is essential only that they willnot contaminate the bath during electrolysis. With respect to thecathodes, there is less danger of contamination-and a Wide variety ofmaterials or combina- 2 tion of materials may be used such as graphite,lead, copper, nickel, brass, aluminum, tin and various stainless steels.

In accordance with the invention the electro-' lyte contains thefollowing elements in an amount within the ranges shown:

Grams per liter Sodium 20 to 40 Ammonium 30 to Chromium 18 to Sulfateion is also present in an amount at least stoichiometrically equivalentto the ammonia, sodium and. chromium.

It has also been found that when the amount of chromium present exceedsabout 30 grams per liter, the concentration of the sodium must be lessthan that of the chromium in order to obtain satisfactory results.

As a typical example of an electrolyte prepared in accordance with theinvention, which is given merely by way of illustration, satisfactoryresults have been obtained by utilizing an electrolyte containingapproximately 32 grams of chromium per liter, with sulfate ion presentin the necessary stoichiometric amount.

The method of forming the electrolyte is not critical except in so faras it is necessary that the solution be substantially free of lead,aluminum and magnesium. The aqueous solution can be made conveniently,for example, by dissolving the necessary amounts of sodium sulfate,ammonium sulfate and chromium sulfate. In general, it is preferred touse chromium. ammonium alum, which is readily available in pure form;although it has been found that potassium alum may be substituted for orused in combination with the ammonium alum. The preparation of this alumin a form suitable for use in the present invention, is disclosed in mycopending application Ser. No. 645,474, filed February 4, 1946. Thesolution is stabilized in the green modification by heating for a shortperiod at a temperature above C.; but not substantially above theboiling point at atmospheric pressure.

Substitution of some of the sulfates in the electrolyte may be made withchlorides; provided enoughsulfate is present to form a complex of thealum type.

At the beginning of electrolysis the chromium present will besubstantially all in the trivalent state (Cr+++). As electrolysisproceeds, however, divalent chromium (Cr++) is formed. Ingeneral, thechromous ion will not accumulate to an extent beyond about20 grams perliter, when the total chromium is about 35 grams per liter. Satisfactoryresults are obtained when the chromous ion has been formed byelectrolysis to a substantial extent of say, about 45% of the totalchromium present. If desired, the formation of chromousion may beexpedited. by;omitting the ammonium-.sulfate from the bath during 'theinitial period of electrolysis. Sufficient chromium should be present,however, so that the amount of chromous ion will not exceed 95% of thetotal chromium.

It has been found that the pI-Ibfithe electro lyte is anotherdeterminingafactor inathesuccessful carrying out of thepresentinvention'. It:

is preferred to adjust the pH of the bath to within the range of about1.8 to 2.2.. This maybe accomplished without the addition"ofanynew:

radicals, for example, by the addition of sulfuric acid. pH of theelectrolyte shbuld be within the range Of) 1.0;23

'Ihe:.-temperature= of'the batiris also. a very important factor inaccomplishing; the aimsofthepresent invention. At temperatures; below27-" 0.; if maintainedfor any substantial period of time, thereisartendency to form insoluble; violet chromium salts. At temperaturesabove about-45"'C., there is a decrease in current effici'ency andthenature of theplate is also adversely affected;

The-current density utilized in thecell also has-a. controlling.influence -upon. the; successful operation of thermethodofthepresent-inven-.- ti'on. In general, a-. cathodic current density. in.therange of. about.3 l to 16.0: amperes per squarev foot .is necessaryto obtainagood. plate. Ifl-thepH isf-rom 2.51 to 2.8,. the preferred.current density. is 30lt'o1 60- amperes. per square foot; from pH 2.3 to1.8, current density, 50. to.100 amperespersquare foot; and fromlpH.1.0Lto 1.8, current density. 100' to- 160 amperes. per square foot..

Dhringz the. operation. of the .cell, it hasv been found, in accordancewiththe present invention..that a.substantiallincreasein current efiisciency and agreatly: improved. structure. of the depositisbbtained. by.adding a. reduced sulfur compound suclias. sulfur. dioxide, .or water.-sol'uble sulfites,. h-yposulfites, sulfamites Xanthates, thionates,.thiosulfates, sulfidesandother sulphuncompound's-in. which the-sulfurhas :a valence of less than .6. The amount required to efiectlsuchimprovement. is .very small and pref-e erably about l2 1ilri mols:per..liter. of; any of these: compounds.

An excess of reduced sulfur. compounds is; to be: avoided. Since; the;reduced: sulfur com-.- pounds-are. destroyed during, electrolysis,-. itis necessary to replenish themairnount ofthese compounds in: thecatholytez This:-is-preferably done continuously,- byadd-ing,.forexample, 0.00002? 0.0002-.-mols.-per liter, per hour;of'sul-fite ionto the. catholyte Even: when added: ina small amount; ithas-beerrfound that after. several hours of.- cyclic operation; theaccumulationof sulfur= compoundsaham an adverse. effect. on thecurrent-.efiiciency and structure of the deposit. It is an-- advantage-0f theapresent invention, however, that such excess may be effectivelyremovedwithout adversely afiecting; the electrolyte: In accordance:with. theinvention, this; is a-cconlplisl'iedi periodically by heatingthe electrolyte-fwith ammonium persulfate:orihydrogen peroxideeat. a.convenient point irr thecircuit,

In general, for satisfactory results; the:

required is small. For example, with the preferred addition agentammonium persulfate, only about 2 grams per liter is required to removeexcess reduced sulfur compound. After such treatment, the electrolyte issuitable for re-use in the cell without further treatment.

It will beapparent that the present invention may be practiced, ifdesired, in the usual batch type of operation. For example, in plating,the bath could be used with an insoluble anode to plate out Cr withinthe concentration range 18-60. grams per liter. A soluble chromiumanodecouldals'o. be used to maintain the chromium concentration.However, it is preferred, and-an advantage-of the invention, to operatethe process continuously. This may be accomplished' by continuously orintermittently replacing part or all of the spent electrolyte withfreshsor: replenished electrolyte prepared as set forth above, or byreplacing the chromium, whiohcwas =removed byelectroplating, intheelectrolyte, for example, in accordance. with...the.

processesset forth in my copending applications- Ser. No. 645,474, filedFebruary 4,. 1946,. and Ser. No. 578,549, filed. February 17, 1945,.thelatter now: abandoned.v

The following specific examples are given..of-. the carrying out of. thepresent. invention, merelyby way of illustration,. to. assist in.. the.understanding of theinvention-and are .notto: be construed aslimiting-the scopeohthe inyen: tionz.

As an example of the practiceof theinvention, reference ismade to.- atest involving. five. weeks of. continuous electrolysisusing. onecathode. at aitime; Each cathode remained: in. the (38112148 hours. Theoriginal electrclytecontained. about 36 grams of chromium per; liter, 33gramsiof sodium per liter and. 42 grams: of. ammonium. per liter. It washeated to abouteiiil C. for about. thirty minutes and cooled. Thesolutionlwas circulated through the cathode compartment of: a cell andthe chromium was replenished:-.-.to maintain the chromium contentabout'35'grams per liter. About: 0.01 ofagram of. ammonium sulphite perliter was added per hour.- The-av,- erage-pl-F of; theeatholytewas:'1-.85 and the =anolyte contained about250 grams of sulphuric acid'per liter." Afterza few hours of operation; the ratioof chromoustochromic: ions was: about 1 to 1. ThBT-SOhltiOni contained not more thanmilligrams per liter-oft.magnesium'and: alumie num and substantiallyno'otherimpurities. The

average catholyte temperature Was 29.5 C.:.ancl

its density was about 1.26: The current density was .maintained' atabout amperes per. square foot;v On the; average, three pounds-of. denseand satisfactory chromium was obtainedaeach 24 hours at a" greatlyincreased current efficiency with anexpenditure' of about 8 kilowatthours per pound of metal. This depositedchromium was readily strippedfrom the cathode plateby hammering and passing 'a thin steel blade undertheideposit'.

m a similar run, in which no. ammoniumsul phite was added, the depositswerehard and bright. They peeled. readily from the cathode; thuscausing. somedifiiculty by prematuresepe aration. Then, 0.3 to 0.4 of agram of ammonium sulphite. was added to 30 liters. of'catholyte... Thecurrentefiiciency was. increased. by the-hourly addition of this."amount ofa-mmoniumsulphite and'was. maintained substantially; constant.-at

outside-thewcell..- Theamount ofiadditionr agent diificultypwas,experienced. from prematureepeele ing, and the current efficiencyincreased substantially.

It will be appreciated that the invention Will be susceptible of manyvariations and modifications by one skilled in the art without departingfrom the spirit of the invention, and all such variations andmodifications are intended to be included within the scope of thefollowing claim.

What is claimed is:

.A method of electrowinning chromium in a compartment cell comprisingforming an electrolyte having a pH range of 1.8 to 2.3 and consisting of18-60 grams of chromium per liter, 30-50 grams of ammonium per liter,20-40 grams of sodium per liter, the amount of said sodium not exceedingthe amount of said chromium when more than grams of chromium per literare present, and sulfate ion in an amount at least the stoichiometricequivalent of the chromium, sodium and ammonium, heating the electrolyteto a temperature of approximately C. to stabilize the chromium sulfatein the green modirfication, and electrolyzing said electrolyte at acathodic current density of about 50 to amperes per square foot whilemaintaining the electrolyte at a temperature in the range ofapproximately 27 C. to 45 C., and during the elec- REFERENCES CITED Thefollowing references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 526,114 Placet et a1. Sept. 18,1894 FOREIGN PATENTS Number Country Date 19,344 Great Britain Sept. 26,1890 22,855 Great Britain Nov. 19, 1891 18,743 Great Britain Aug. 5,1899 OTHER REFERENCES Rogers et al., Transactions of the ElectrochemicalSociety, vol. 64 (1933), pages 299-304.

